1. Field of the Invention
The present invention relates to a technique for digital radio space diversity combining and, more particularly, to a digital radio space diversity combiner which includes two branches wherein separate multipath signals received from a remote source on separate antennas are propagated. The branch signals are combined, and the output signal is used for obtaining noncoherent spectrum measurements which are used to account for both dispersion and noise in controlling the relative amplitudes and phases of the signals propagating in the two branches.
2. Description of the Prior Art
In a terrestrial digital radio link, frequency selective fading caused by multipath propagation presents the major threat to system availability. Efforts to reduce channel dispersion, and thus to increase channel availability, typically center on the use of adaptive equalization and/or dual-branch space diversity. In this regard see, for example, U.S. Pat. No. 4,261,056 issued to W. T. Barnett et al on Apr. 7, 1981 relating to an equalizing signal combiner.
Most conventional space diversity schemes use either selection switching or so-called "in-phase" combining of the diversity branches. The latter approach concentrates on maximizing the combiner output power rather than on minimizing channel dispersion. Recently, however, techniques have dealt with "out-of-phase" combining, which reduces output dispersion by suitably adjusting the relative phases between the two branches. In this regard see, for example, the article "Performance of 16-QAM Digital Radio System Using New Space Diversity" by S. Komaki et al in ICC 80 Conference Record, Seattle, Wash., June 8-12, 1980, Vol. 3 at pages 52.2.1-52.2.6. Such approach can substantially eliminate dispersion for specific two-path propagation situations, but not under more general and realistic conditions.
Adaptive cross-polarization interference cancellation and adaptive signal interference cancellation arrangements are disclosed in U.S. Pat. Nos. 4,283,795, issued to M. L. Steinberger on Aug. 11, 1981 and 4,320,535 issued to D. M. Brady et al on Mar. 16, 1982, respectively. In such arrangements the combined signal is used to generate feedback signals which control the phase and amplitude of the interfering signal to provide interference cancellation.
The problem remaining in the prior art is to provide a space diversity combiner which provides adaptive control of the relative amplitudes and phases of the two branch gains prior to combining and search strategy that simultaneously accounts for both dispersion and noise.